Sir Joseph Barcroft (26 July 1872 in Newry, County Down – 21 March 1947 in Cambridge) was a British physiologist best known for his studies of the oxygenation of blood.

He received his degree in Medicine and Science in 1896 from Cambridge University, and immediately began his studies of haemoglobin. In May, 1910 he was elected a Fellow of the Royal Society and would awarded their Royal Medal in 1922 and their Copley medal in 1943. He would also deliver their Croonian Lecture in 1935.^[1]

In the course of his research, he did not hesitate to use himself as a test subject. For example, during the First World War, when he was called to Royal Engineers Experimental Station (near Salisbury) to carry out experiments on asphyxiating gas, he exposed himself to an atmosphere of poisonous hydrogen cyanide. On another occasion he remained for seven days in a glass chamber in order to calculate the minimum quantity of oxygen required for the survival of the human organism, and another time he exposed himself to such a low temperature that he collapsed into unconsciousness.

He also studied the physiology of oxygenation at extreme altitudes, and for this purpose he organized expeditions to the peak of Tenerife (1910), to Monte Rosa (1911), and to the Peruvian Andes (1922).

From 1925 to 1937 he held the chair of physiology at Cambridge. His final research, begun in 1933, concerned fetal respiration. Between 1902 and 1905 he was a Governor of Leighton Park School, the Quaker School in Reading.

During the first years of the Second World War he was again summoned to Porton Down to consult on chemical weapons.